In order to improve a driving safety, various control systems, e.g., ABS, are equipped to automobile in recent years. The ABS is comprised of a wheel speed sensor to detect a rotation speed of a wheel, an electric control unit (ECU) to measure a signal generated by the wheel speed sensor and an actuator that is operated by an output signal from the ECU, wherein the brake is controlled by an operation of the actuator.
The signal from the wheel speed sensor is transmitted to the ECU via an ABS sensor cable. Normally, the ABS sensor cable consists of twisted pair of insulated wires, the external circumferential surface thereof being covered by an intermediate filler material for securing the integrity of the circular cross sectional structure of the cable, and then covering the external circumferential surface of the intermediate filler material with a sheath. FIG. 1 is a cross sectional view of an embodiment of a halogen free flame retardant cable in accordance with the present invention that is cut along a surface perpendicular to the longitudinal direction of the cable. A conventional ABS sensor cable also has a similar structure.
The wheel speed sensor is installed near the wheel and exposed to a severe environment, such as water splash and ice coating and is required that the seal between the ABS sensor cable and wheel speed sensor is waterproof. Therefore, after the ABS sensor cable is connected to the wheel speed sensor, the entirety thereof is molded with a plastic resin such as polybutyleneterephthalate (PBT), nylon or the like.
Preferably, a certain material having a heat adhesion property to the molding material such as PBT or nylon may be used to manufacture the sheath covering the cable to provide a high seal performance without having to use a seal member such as an O-ring or the like, thereby reducing the manufacturing cost thereof, while ensuring the waterproofness. In addition, the sheath material is required to have an abrasion resistance, flexibility, superior mechanical strength and the like. A mixture of a thermoplastic polyurethane elastomer and a thermoplastic polyester elastomer, due to its excellent heat adhesion property to the molding material, the mechanical strength and the like, has been used in manufacturing the sheath (see, e.g., Japanese Patent Laid-open Application No. 10-177818, Claim 1).
On the other hand, since the wire and the cable for use in an automobile are required to have a flame-retardant property, the ABS sensor cable need be made of a material having a flame-retardant property. A mixture of a thermoplastic polyurethane elastomer and a thermoplastic polyester elastomer is normally flammable and cannot meet the flame-retardant property requirement stipulated by the JASO standard for a wire product for use in an automobile. As an effort to overcome the deficiency, therefore, a halogenated flame retardant such as a chloride or bromide type has been added to the thermoplastic mixture employed to manufacture the sheath.
With the increasing awareness of environmental problems in recent years, it is considered that utilization of halogenated flame retardants for flame retardation of polymers is not so suitable selection for realizing environmental friendly society. Because polymer wastes that contain halogenated flame retardant generate harmful substance such as hydrogen halide, and sometimes generate dioxin in certain condition when they are disposed by incineration. From the view point of above mentioned background, the development of a halogen free flame retardant heat adhesion type of ABS sensor cable is demanded. Despite various efforts, however, the industry has failed to produce a halogen free flame retardant having a flame-retardant property compatible to that of a halogenated flame retardant. Also, the use of a large amount of a flame retardant to ensure a high level of flame-retardant property tends to deteriorate the heat adhesion property, abrasion resistance and the like of the sheath.